I. Field of the Invention
The invention presented relates to pseudonoise (PN) sequence generators. More particularly, the present invention relates to a method and an apparatus for generating PN sequence with each clock pulse by computing their bits in parallel.
II. Description of the Related Art
The Telecommunications Industry Association has standardized a method for code division multiple access (CDMA) communications in the IS-95 family of interim standards, entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System.xe2x80x9d In addition, the Telecommunications Industry Association in its submission to the International Telecommunications Union, entitled xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submission,xe2x80x9d describes proposed CDMA system that would be able to support higher data rates and higher capacity. Both in the IS-95 standard and in the cdma2000 proposal, the transmitted waveform is modulated in accordance with a pseudonoise spreading sequence.
The use of a pseudonoise sequence with appropriate autocorrelation characteristics is essential to the operation of a CDMA system in which multipath components are present. The generation and employment of pseudonoise sequences are described in detail in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention, and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention, and incorporated by reference herein.
The aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459 describe the use of a pilot signal used for acquisition. The use of a pilot signal enables the remote user to acquire local base station communication system in a timely manner. The remote user gets synchronization information and relative signal power information from the received pilot signal. U.S. Pat. Nos. 5,644,591 and 5,805,648, both entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,xe2x80x9d describe a novel and improved method and apparatus that reduces the remote user forward link acquisition time. Both patents are assigned to the assignee of the present invention and are incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a remote user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention, and incorporated by reference herein.
In CDMA communications systems, a pilot signal is transmitted that allows a receiver to coherently demodulate the received signal. Within demodulator of such receivers is a channel estimate generator, which estimates the channel characteristics based on the pilot signal transmitted with values known to both the transmitter and the receiver. The pilot signal is demodulated and the phase ambiguities in the received signal are resolved by taking the dot product of the received signal and the pilot signal channel estimate. An exemplary embodiment of a circuit for performing the dot product operation is disclosed in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPILOT CARRIER DOT PRODUCT CIRCUIT,xe2x80x9d assigned to the assignee of the present invention, and incorporated by reference herein.
The invention presented is a novel method and apparatus for generating a PN sequences with an arbitrary number of bits, where the number of bits is provided in parallel with each clock pulse. This allows the sequences to be generated at high speed when needed, and allows parallel processing in the acquisition and demodulation processes. The invention describes in detail generation of PN sequences as standardized for the IS-95 communications systems. As proposed in the IS-95 standards, the pseudonoise spreading sequences are maximal length sequences that are capable of being generated using linear feedback shift-registers (LSFRs). Using a linear feedback shift-register, the PN sequences are computed one bit with each clock pulse.
In the invention, the initial PN states are loaded into registers of a parallel PN generator, which immediately generates the next n bits of the PN sequence, where n is an arbitrary number dependent on performance required. In addition, the present invention provides a method of determining the register states of the parallel PN generator an arbitrary number of cycles in the future. Thus, the present invention takes the present state of the registers of the PN generator and outputs the next n bits of the generator. In addition, the PN generator of the present invention receives the present state of the PN generator and outputs the state of the PN generator n bits in the future. In this fashion, the entire PN sequence can be continuously generated.
It will be understood by one skilled in the art that although the present invention is directed toward the generation of a psuedonoise sequences compliant with systems standardized by the Telecommunications Industry Association, the teachings of the present invention are equally applicable to the generation of other psuedonoise sequences such as, the orthogonal Gold code sequences proposed for use in the W-CDMA, proposals to the International Telecommunications Industry Association, proposals by the European Telecommunications Standards Institute (ETSI), and the Association of Radio Industries and Business (ARIB).